The Doors, Back Door Man.
Second Hint:
With Gay Men, it replaces Home Plate, since there is no Home Plate between two Gay Guys. Something is missing from the equation and can only be provided by the Opposite Sex.
That's the best I can do with this being a Family oriented Site.
Sodomy is bad and unhealthy male or female.
Back in the day I think Back Door Man meant they guy you were cheating with (sneaking out the back door as your husband comes in the front).